My concern is isotropic backgrounds: they state that they subtracted the CMB,
but what about

(i) the background from radio galaxies etc, which has a steeper spectral
index than the Galactic synchrotron

(ii) instrumental offsets of the surveys.

Both of these are well documented in the literature but are not mentioned in this paper as far as I can make out.

Their PCA Component 1 contains as they say 'total stuff' but this is then a combination of Galactic and extragalactic emission ?
Would it not be advantageous to add an isotropic component to their PCA analysis.?

Is it because a PCA analysis is concerned only with variations ?
But how can it then claim to predict the actual sky at each frequency ?